1. Field of the Invention
The present invention relates to an ink jet recording apparatus, and more specifically to an ink jet recording apparatus for recording dots on a recording medium by jetting a liquid ink obtained by dispersing color material components in a solvent, as ink drops.
2. Description of the Prior Art
Conventionally, an ink jet type printer for forming recorded dots by jetting a liquid ink onto a recording medium has been well known. In the ink jet recording apparatus of this type, since there exists such advantages that noise is low, as compared with the other recording method and further since the development and fixing are both not required, this recording apparatus has been noticed, in particular as a recording technique for recording dots on plain paper.
At present, a number of ink jet recording methods have been so far proposed. For instance, Japanese Published Examined (Koho) Patent Application Nos. 56-9429 and 61-59911 disclose a method of jetting ink drops by pressure obtained by heat generated by a heating element. On the other hand, Japanese Published Examined (Koho) Patent Application No. 53-12138 discloses a method of jetting ink drops on the basis of a mechanical pressure pulse generated by a piezoelectric crystal element.
In particular, the former technique is a typical example of a multi-nozzle type recording apparatus for recording a plurality of dots by arranging a plurality of nozzle in parallel to each other. Further, as the recording head used for these techniques, a serial scanning type head mainly mounted on a carriage and repeating the scanning operation in a direction perpendicular to a moving direction of recording paper is put to a practical use.
On the other hand, in such a type that a multi-nozzle is arranged on a long recording head having a length the same as the width of recording paper, instead of mounting the recording head on the carriage, although the moving parts can be reduced and thereby the recording speed can be increased, it is not easy to manufacture a line-scanning type recording head as described above, in practice.
In other words, in the multi-nozzle type, there exist such a problem in that ink is easily enriched locally due to evaporation or volatilization of the solvent. Therefore, when independent slender nozzles are arranged to obtain a high resolution required for recording, these slender nozzles tend to be clogged. In particular, in the case of the type using steam pressure, there exists a problem in that insoluble substance is easily adhered onto the nozzle due to thermal or chemical reaction of the ink. On the other hand, in the type using a piezoelectric crystal element, since the ink flow paths are rather complicated, these ink flow paths are easily clogged.
In the case of the recording head of serial scanning type which requires several tens to one hundred and several tens of nozzles, however, owing to the advance of the recent technique. it is not difficult to suppress the frequent nozzle clogging. In the case of the line scanning type structure in which several thousands of nozzles are arranged, on the other hand, since the probability of nozzle clogging is still high, there exists a problem in that the reliability is low.
On the other hand, in the case of the ink jet type, another problem arises in that it is very difficult to increase the resolution. In more detail, in the case where stream pressure is used, it is very difficult to form an ink drop whose diameter is less than 20 .mu.m. Here, since the ink drop having a diameter of 20 .mu.m is recorded as a recorded dot having a diameter of about 50 .mu.m, it impossible to increase the resolution more than this value in practice. Further, in the case where a piezoelectric crystal element is used, since the structure is rather complicated, there exists another problem in that it is difficult to manufacture a recording head of high resolution due to the restriction of manufacturing technique.
To overcome the above-mentioned various problems, a method of jetting ink or color material components contained in the ink, from an ink liquid surface, by use of an electrostatic force obtained by applying a voltage to a thin film electrode array has been proposed. For instance, Japanese Published Unexamined Patent Application Nos. 49-62024 and 56-4467 disclose a method of jetting ink by use of electrostatic force. Further, Japanese Published Unexamined Patent Application No. 7-502218 discloses a method of jetting ink containing charged color material components, in order to increase the density of the color material.
In these methods, since a slit-shaped nozzle or a nozzle-less structure can be adopted, without need of an independent nozzle for each dot or ink flow path partitions, the nozzle clogging (which is a serious problem caused when the recording dots are arranged in a straight line) can be prevented and restored effectively. In particular, in the method of using the ink containing charged color material components and jetting the ink containing a high density color material, ink particles having very small diameters can be formed and jetted stably, this method is suitable for high resolution recording apparatus.
(1A) Further, in the above-mentioned prior art ink jet recording apparatus for jetting ink drops by use of electrostatic force, the ink drops can be jetted when a pulse voltage varying according to image signals are applied to the electrode array. Here, the case where ink drops are jetted from the electrodes by applying the pulse voltages to only the alternately arranged electrodes of the electrode array will be considered. This corresponds to the case where vertical stripes are recorded along the sub-scanning direction. In this case, since an electric field can be generated from an electrode to which the pulse voltage is applied to the adjacent electrode to which no pulse voltage is applied, the color material components are moved by the electric field generated between the two adjacent electrodes from the voltage-applied electrode to the non-voltage-applied electrode. Therefore, when this status continues, since the density of the color material components of the ink becomes non-uniform on the respective electrodes, this causes a non-uniformity in the density of the recorded image. That is, since the quantity of the charged color material components moved by the electrostatic force lacks on the respective electrodes, the ink jet operation becomes unstable, so that the density of the recorded image is not only non-uniformalized but also the ink drops cannot be jetted at the worst. PA0 (1B) Further, when the above-mentioned status continues for a long time, since the color material components are deposited onto the electrodes to which no pulse voltages are applied and thereby the viscosity of the ink is increased, electric charges are moved between the color material components and the electrodes, so that the electric charges of the color material components are lost. Under these conditions, since the color material components on the electrode to which no pulse voltage is applied cannot be moved by the electric force, the color material components adhere onto the electrode. As described above, the color material components adhered onto the electrodes causes a change in the electric field distribution or obstructs the movement of the other color material components, with the result that a continuous ink supply is not obtained. PA0 (1C) Further, in the prior art ink jet recording apparatus for jetting color material components as ink drops by electrostatic force, in order to jet the ink drops stably, it is necessary to form an ink thin layer at the ink drop jetting position stably, and further to supply ink drops containing sufficient color material components at the ink jetting position. In the prior art ink jet recording apparatus, however, since the construction is such that the ink drop jetting position is determined in the ink flow path and the ink reaching this ink jetting position can be jetted onto the recording medium by the electrostatic force, the thickness of the ink layer is easily changed due to a slight change of the ink supply pressure or the influence of the atmospheric pressure, with the result that there exists a problem in that the ink drops cannot be jetted stably and thereby the recording of a stable ink density cannot be obtained. PA0 (1D) Further, in the prior art ink jet recording apparatus for jetting color material components as ink drops by the electric field, in order to maintain the continuity of the ink drop jetting, since auxiliary electrodes are provided in addition to recording electrodes in order to control the inflow or outflow of the color material components from an ink holding portion, when the density of the color material increases within the ink holding portion, the color material is easily adhered onto the ink holding portion. As a result, there exists a problem in that the color material components cannot be supplied smoothly. On the other hand, after power of the recording apparatus has been turned off, since such a probability that the color material components adhere onto the ink holding portion is relatively high, there arises a problem in that the ink supply capability is not sufficient whenever the ink supply has been restarted. PA0 (2A) With these problems in mind, therefore, it is a first object of the present invention to provide an ink jet recording apparatus for jetting cohered color material components contained in ink onto a recording medium by electrostatic force, which can uniformalize the density of the color material components on each of the electrodes arranged on a head substrate and further can prevent the color material components from adhering onto the electrodes to stabilize the recording operation. PA0 (2B) Further, a second object of the present invention is to provide an ink jet recording apparatus for jetting cohered color material components contained in ink onto a recording medium by electrostatic force, which can prevent the charged color material components from being scattered from the ink drop jetting position to obtain a stable recording without causing the density non-uniformity. PA0 (2C) Further, a third object of the present invention is to provide an ink jet recording apparatus for jetting cohered color material components contained in ink onto a recording medium by electrostatic force, which can realize a stable density recording without causing nozzle clogging on the basis of a stable electrostatic force. PA0 (2D) Further, a fourth object of the present invention is to provide an ink jet recording apparatus for jetting cohered color material components contained in ink onto a recording medium by electrostatic force, which can prevent the color material components from adhering to the ink holding portion or onto an insulating substance in the vicinity of the electrodes and further can supply the ink under excellent conditions by optimizing the ink condition whenever the apparatus power is turned on again, even after the power has been turned off and further kept unused for many hours. PA0 (3A) To achieve the first object, in the ink jet recording apparatus according to the present invention, for supplying ink containing color material components dispersed in a solvent onto a head substrate and by jetting an ink drop containing at least the color material components toward a recording medium on the basis of an electrostatic force applied to the color material components of the ink, the recording apparatus comprises the voltage applying means for applying voltages to a plurality of electrodes arranged on the head substrate to stir the color material components contained in the ink supplied to the head substrate. In more detail, when at least first and second electrodes are arranged on the head substrate, the voltage applying means applies voltages to the first and second electrodes in such a way that a potential difference whose potential level relationship is reversed at a predetermined period between the first and second electrodes can be applied. Further, when the ink drop is not jetted, the voltage applying means applies the voltages to the first and second electrodes in such a way that a first potential difference whose potential level relationship is reversed at a predetermined period between the first and second electrodes can be applied; and when the ink drop is jetted, a second potential difference different from the first potential difference can be applied between both electrodes. PA0 (3B) To achieve the second object, in the ink jet recording apparatus according to the present invention, for recording an image by jetting ink drops containing at least color material components dispersed in ink and charged to a predetermined polarity in a solvent on the basis of electrostatic force, the basic features thereof are as follows: a plurality of electrodes are arranged at positions where the ink is to be supplied, and voltage applying means is provided for applying voltages to a plurality of the electrodes in such a way that a potential difference between the first electrodes the nearest to the ink-jet position and the recording paper becomes lower than the potential difference between the second electrodes adjacent to the first electrodes and the recording paper according to image signals. Here, a plurality of the electrodes are formed on the head substrate to which the ink is supplied or the electrodes of coaxial type. PA0 (3C) To achieve the third object, in the ink jet recording apparatus according to the present invention, for recording an image by jetting the ink having the color material components dispersed in a solvent contained in the ink toward the recording medium on the basis of electrostatic force, the ink circulating system for circulating the ink is mounted on the head substrate having the array electrodes for applying an electrostatic force to the color material components in the ink; the slit-shaped nozzle branched from the ink circulating system is formed on the head substrate; and the ink drops are jetted toward the recording medium through the slit-shaped nozzle. Therefore, it is possible to stabilize the jetting of the ink drops. PA0 (3D) To achieve the fourth object, the ink jet recording apparatus according to the present invention comprises the ink holding means for holding the ink obtained by dispersing the color material components in insulating solvent; the recording electrodes arranged in the ink holding means to jet the color material components dispersed in the solvent by electrostatic force; and the ink stirring means arranged in the vicinity of the recording electrodes to stir the ink. (4D) Further, the ink jet recording apparatus comprises the ink holding means for holding the ink obtained by dispersing the color material components in insulating solvent; the recording electrodes arranged in the ink holding means to jet the color material components dispersed in the solvent by electrostatic force; the carrying electrodes for carrying the ink within the ink holding means in the ink supplying direction and in the ink returning direction; and the ink stirring means arranged in the vicinity of the recording electrodes to stir the ink.